Note: Descriptions are shown in the official language in which they were submitted.
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GRANULATION OF METALLURGICAL SLAG
Technical field
[0001] The present invention generally relates to granulation of slag from the
metal industry and more particularly from the iron industry.
Background Art
[0002] Conventionally, metallurgical slag is granulated in water.
[0003] Water quenching ensures fast solidification of the metallurgical slag,
which, in the case of blast furnace slag, is a necessary condition for
obtaining a
valuable product. A water jet is firstly used to fragmentize the hot liquid
slag
stream into very small particles and to transfer them into a water bath. The
energy
form the hot slag is withdrawn through direct contact between the hot liquid
slag
and the water. As this has to happen at ambient pressure, the temperature of
the
slag is immediately lowered to a temperature level of below 100 C.
[0004] During the water granulation process, the sulfur contained in the hot
liquid
slag reacts with the water and generates sulfur dioxide (SO2) and hydrogen
sulfide
(H2S). The quantity of these toxic and malodorous gases is mainly dependent on
the chemical composition of the slag and the granulation parameters.
[0005] The concentrations of both sulfur dioxide and hydrogen sulfide may be
too
high with regard to the environmental legislation so that measures have to be
taken to lower these emissions to an acceptable concentration.
[0006] In the state of the art (see e.g. EPO 573 769, LU 88 441 and/or LU 91
424,
from Paul Wurth S.A.) a condensation tower is erected above the granulation
area.
In the condensation tower, both SO2 and H25 are condensed with the water vapor
and hence recombined with the water to form sulfuric acid (H2504). The water
is
pumped into a cooling tower in which, due to contact with the ambient air
under
turbulent conditions, sulfur is partially transferred from the hot process
water to the
cooling air. Due to the large amount of cooling air, however, only small
concentrations of SO2 and H25 (below 1 mg/Nm3) are measured.
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[0007] JP2002308655 discloses a method to mix hot liquid slag with iron-
containing fine powder to adjust and control the density of the slag. The
addition of
oxidized iron powder decreases the nitrogenated particles trapped in the
molten
slag and thus allows to obtain a high density slag independent of the tapping
temperature of the slag.
[0008] CN 101 545 018A describes a process and a device wherein the ladle slag
first enters the drum slag area from the funnel. There are steel balls in the
slag
area, and as the drum turns, the slag is cooled by the steel balls. The heat
of the
molten steel and the slag is quickly removed by the steel balls, and the slag
is
plasticized. After solidification, it is carried by the rolling steel balls
into the water
cooling area. The slag and the steel balls are simultaneously cooled by the
water.
Then it is carried to the outer drum body through the drum's lattice-like
grate bars,
and the slag is cooled once again by the water in the outer drum and finally
discharged. During this process, due to the effect of the fast rolling of the
steel
balls, the slag and steel are separated. The amount of cooling water is 1.25T
to 2T
per ton of slag. The steel ball rolling speed is 0.5 m/s to 2 m/s. The speed
of the
rolling balls makes sure the steel balls are not enveloped in the cooling
molten
steel. A suitable steel ball rolling speed guarantees that the granulation of
the slag
and the separation of the molten steel. It prevents lumping in the cooling
process.
Technical problem
[0009] It is an object of the present invention to provide a method of
granulation
of hot liquid slag, which minimizes the formation of sulfurous emissions
during the
granulation of hot liquid slag.
General Description of the Invention
[0010] To achieve this object, the present invention proposes a process for
granulation of hot liquid slag wherein the hot liquid slag is mixed with
metallic
particles so as to form a solidified, vitrified slag cake mixed with said
metallic
particles and wherein said slag cake mixed with said metallic particles is
discharged in a water bath for further cooling, the hot liquid slag is poured
first into
a mold and then the solid metallic particles are poured into the mold
containing the
hot liquid slag. The hot liquid slag and the solid metallic particles are
mixed so as
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to form a solidified, vitrified slag cake with the solid metallic particles
entrapped
therein.
[0011] The advantage of using discrete, solid, inert, metallic particles
submerged
into the liquid slag is that the heat transfer is very efficient and quick so
that the
[0012] It has been found that during the traditional water or wet granulation
where
the hot liquid slag is brought into contact at about 1500 C with water (below
100 C) and fragmented by a water jet into small slag particles, a vapor layer
is
disintegration of sulfides (CaS, FeS, MnS) in the vapor layer between a slag
particle and the liquid water.
[0014] This effect is minimized by the process according to the present
invention
in two ways:
20 1. The temperature of the slag in the present process is much lower
than
in the traditional water granulation (about 700 C vs about 1500 C) when it
comes
in contact with the water. The period of time where the Leidenfrost effect can
take
place is thus much shorter and therefore the quantity of sulfurous compounds
formed is much lower.
25 2. The slag is already fully vitrified i.e. solidified when it comes
in contact
with the water. The water thus does not break the slag into small particles of
about
1 mm as in the traditional water granulation, where a stream of hot liquid
slag is
disrupted by the impact of a water jet at high speed. As a consequence, the
available surface per kg of slag where the Leidenfrost effect can take place
and
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[0015] The combination of these two effects reduces the quantity of sulfurous
compounds generated during the granulation process greatly.
[0016] When reference is made in the context of the present document to a
dimension of a piece of slag cake this is understood to mean the diameter of
the
sphere that has same weight as a given particle.
[0017] A further advantage of the present invention is that the metallic
particles
are easily separated from the slag after cooling and may be reused in the
process.
[0018] Additionally, as the metal particles are inert and do not react
chemically
with the slag, the chemical composition of the slag is not affected.
[0019] According to a preferred embodiment, the mold is first filled up to
preferably about one third of its height with liquid slag and then the solid
metallic
particles are introduced into the mold.
[0020] The mold in which the hot liquid slag and the solid metallic particles
are
poured is preferably integrated in a troughed belt conveyor.
[0021] According to a preferred embodiment, the volume of slag in each mold is
measured and the quantity of solid metallic particles is adapted to the volume
of
hot liquid slag in the mold so as to obtain an efficient and quick
solidification and
vitrification of the slag.
[0022] The solid metallic particles are preferably dropped from a height of
about 1
to 3 m to obtain a quick and efficient mixing of the slag and the solid
metallic
particles. The exact height i.e. the exact amount of energy required for the
particles to penetrate the liquid slag to the desired depth depends on the
composition of the slag, the temperature of the slag, the density and the
diameter
of the solid metallic particles etc.
[0023] In order to obtain a good distribution of the solid metallic particles
in the
slag cake, the solid metallic particles are preferably distributed over the
mold by a
vibrating chute, a static device with fixed metal rods or a similar
distributing device.
[0024] The slag cake containing the solid metallic particles is preferably
broken
down to pieces of slag cake of less than about 150 mm, preferably of less than
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about 100 mm, more preferably of less than about 80 mm and most preferably of
less than about 50 mm before being discharged in the water bath.
[0025] The reduction of the size of the pieces of slag cake containing the
solid
metallic particles may be achieved by discharging the slag cake from the molds
impact plate. The slag pieces are further cooled and washed away by a powerful
water jet. This water jet may be provided by an ordinary Paul Wurth SA
granulation head (such as described in e.g. LU 88 380 and/or EP1 422 299).
These granulation heads may deliver a water jet with about 1000 m3/h water
mass
[0027] The solid metallic particles advantageously have a density of at least
2.5
[0028] The solid metallic particles are preferably spherical so as to have
good
mixing properties and to assure a rapid and efficient cooling of the slag.
[0029] The solid metallic particles preferably have a diameter of at least 5
mm
25 preferably more than 8 mm, more preferably more than 10 mm and most
preferably more than 15 mm.
[0030] The solid metallic particles advantageously have a diameter of less
than
30 mm, preferably less than 25 mm, more preferably less than 22 mm and most
preferably less than 20 mm.
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[0031] The solid metallic particles are preferably made of a metal chosen
amongst the group consisting of iron, steel, aluminum, copper, chrome, their
alloys, as well as their alloys with other metals.
[0032] In practice, it is preferred to use solid steel balls because they are
readily
available in different diameters and because they may be easily separated form
the slag once it is cooled by e.g. by a magnet.
[0033] It was shown that in particular steel balls are suitable as solid,
metallic
particles and readily available. Surprisingly, it was found that dropping the
solid
metallic particles from a certain height gives them enough kinetic energy to
penetrate into the liquid slag and to be distributed evenly throughout the
height of
the slag cake which is formed.
[0034] The amount of kinetic energy needed to achieve a uniform distribution
of
the solid metallic particles throughout the hot liquid slag depends on the
viscosity
of the slag, the nature of the particles and their density as well as on their
diameter.
Brief Description of the Drawings
[0035] A preferred embodiment of the invention will now be described, by way
of
example, with reference to the accompanying drawing in which:
Fig. 1 is a flow sheet of a preferred embodiment of the process according to
the
invention.
Fig. 2 shows a picture of a slag lump comprising steel balls.
Description of Preferred Embodiments
[0036] Fig. 1 shows a schematic view of a preferred embodiment of granulation
installation of hot liquid slag (e.g. from a blast furnace).
[0037] Fig. 2 shows a picture of a piece of slag cake with steel balls.
[0038] Fig. 1 shows a schematic view of a preferred embodiment of granulation
installation 10 of hot liquid slag (e.g. from a blast furnace). Hot liquid
slag 12 at a
temperature of about 1500 C having a density of about 2.7 g/cm3 is transported
in
a slag runner 14 or in a slag ladle (not shown) at a flow rate of about 0.5 to
about 6
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t/min. The hot liquid slag 12 is poured onto a slag caster 16 comprising molds
or
troughs 18 which may be refractory lined. The slag caster 16 may be a trough
conveyor belt.
[0039] The dimensions of the slag caster 16 are chosen to cope with the
expected slag mass flow rates from a blast furnace (not shown). After the hot
liquid slag 12 has been poured into a mold 18 of the slag caster 16, a level
measurement by a radar probe 20 indicates the height of the slag layer in the
mold
18 so as to be able to adapt the mass of solid metallic particles - in this
particular
example steel balls are used - to be added to that particular mold 18. The
slag
height inside the mold 18 does normally not exceed about one third of the
height
of the mold. The height can be adapted depending on the properties of the hot
liquid slag 12 (temperature, viscosity, chemical composition etc.) and on
other on-
site circumstances. The mass ratio between steel balls and the hot liquid slag
12 is
chosen so as to achieve the desired temperature in the mixture. The exact mass
ratio thus depends on the temperature of the hot liquid slag 12 and the
temperature of the steel balls, as well as on the density and thermal capacity
of
the slag. For hot liquid slag 12 at 1500 C and steel balls at 30 C, the
necessary
mass ratio (steel balls / hot liquid slag) would be around 2.4 while the
corresponding volume ratio would be around 0.8 The steel/slag mass ratio of
2.4
ensures an equilibrium temperature of the formed slag/steel cake of about 700
C.
(with a slag temperature of 1500 C and a steel sphere temperature of 30 C).
The
densities as well as the heat capacities of steel and slag have been taken
into
account for this calculation. With the given densities, the mass ratio of 2.4
gives a
volume ratio of 0.83 (steel/slag).
[0040] Each mold 18 of the slag caster is first filled with hot liquid slag 12
and
then advances under a first buffer hopper 22 to be filled with steel balls 24,
which
are dropped into each mold 18. The steel balls 24 are dropped into the mold 18
from a height of about 2 m to obtain a quick and efficient mixing of the hot
liquid
slag 12 and the steel balls 24.
[0041] The steel balls 24 are stocked in a first buffer hopper 22 situated
above
the slag caster 16 and are extracted using a speed controllable screw conveyor
26
or a speed controllable vibrating chute. The steel balls 24 are distributed
evenly
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over the whole surface of the mold 18 onto which they are to be dropped using
a
vibrating chute 28 or a static distribution device. The exact size of the
steel balls
24 is depending on the properties of the slag and other specific conditions at
the
granulation site; however a medium diameter of about 10-25 mm may be suitable
[0042] A water sprayer 30 installed above the molds 18 of the slag caster 16
may
be used to further cool the slag cake if necessary or in case of an emergency.
[0043] The mixture of slag and steel balls will rapidly solidify and form a
solidified,
vitrified slag cake 30 (in less then 10-20 s) and reach an equilibrium
temperature
[0044] After reaching the equilibrium temperature, the slag cake 32 needs to
be
[0045] In the preferred embodiment represented on Fig.1, the slag cake 32
leaves the molds 18 in pieces having an average size of about 800 mm, the
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plate 42 at the end of the cold runner 42 further reduces the pieces of slag
cake 32
to about 20-30 mm.
[0046] The pieces of slag cake 32 are finally discharged of the water bath 44
using a belt conveyor 46. This belt conveyor 46 also assumes the role of
dewatering unit for the pieces of slag cake 32. Given the rather large size of
the
pieces of slag cake 32 and their low porosity as compared to the slag
particles
obtained by traditional wet granulation systems, an efficient dewatering on
the belt
conveyor 46 is achieved. Hence, no additional dewatering units such as
dewatering bins or INBA drums (such as described in e.g. LU 84 644, LU 84 642,
and/or LU 79 466) will be necessary.
[0047] The pieces of slag cake 32, having a medium size of about 20-30mm are
then fed to a crusher 48 where the pieces of slag cake 32 containing the steel
balls 24 are crushed so as to form on the one hand slag particles 50 and
liberate
the steel balls 24 contained in the slag cake 32 and thus insure that an
efficient
separation of the steel balls 24 from the slag particles 50 with a magnetic
conveyor
52 is achieved.
[0048] After the separation of the steel balls 24 from the slag particles 50,
the
slag particles 50 are conveyed by a belt conveyor 54 to a storage area 56,
whereas the steel balls 24 are recirculated to the first buffer hopper 22.
[0049] A screen 58 may be used to eliminate worn or damaged steel balls and to
drop them in a box 60 situated underneath the screen 58. The eliminated steel
balls are replaced with new steel balls and fed from a second hopper 60 to the
first
buffer hopper 22.
[0050] The cooling of the pieces of solidified vitrified slag cake 32 takes
place
through boiling heat transfer and the created water vapor 64 is released
through a
stack 66 situated above the water bath 44.
[0051] A conduct 68 provides make-up water to the water bath 44 to compensate
for the evaporated water.
[0052] A booster pump 70 delivers the process water to the granulation head 40
to produce the water jet 38. The temperature of the water bath 44 will be
around
100 C.
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[0053] A recirculation pump 72 assures efficient extraction of any fines that
may
settle at the bottom of the water bath 44
[0054] The present process efficiently reduces the sulfur emissions to the
environment while granulating hot liquid slag.
5 [0055] Indeed, in addition to the above arguments, the average blast
furnace slag
particle obtained a traditional wet granulation method may be assumed to have
a
diameter of approximately 1 mm. In the method described herein, the pieces of
slag cake have an average size of 150 mm before hitting the water jet and
approximately 20 ¨30 mm when falling in the water bath. Accordingly, in this
10 preferred embodiment, the available slag surface for creation of
sulfurous
emissions is reduced by a factor of 20 - 30.
[0056] As seen in Fig.2 representing a picture of a piece of slag cake 32
comprising steel balls 24 obtained according to the process described herein,
the
pieces of slag cake are very compact and have few pores. The total surface of
slag available for the above-described chemical reactions is further reduced.
Moreover, the presence of steel balls yet further diminishes the exposed slag
surface.
[0057] Finally, the reactions leading to the creation of sulfurous emissions
can
only take place at temperatures exceeding 400 C, this being due to thermal
reasons. Indeed, the chemical equations for the creation of SO2 and H2S can be
summarized as follows:
[0058] CaS+3H20 = Ca0+3H2+S02
[0059] CaS+H20 = Ca0+H2S
[0060] Below 400 C, the equilibrium of these reactions is strongly on the left
side,
so that below that temperature there is basically no liberation of sulfur from
the
slag particles.
[0061] Accordingly, sulfur will remain trapped inside the pieces of slag cake
once
this temperature of about 400 C is reached. In traditional wet granulation
systems,
the slag has to be cooled with water from about 1500 C whereas in the
granulation
according to this invention the pieces of slag cake only need to be cooled
with
water from a temperature of about 700 C. Calculations made by the applicant
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show that the required cooling time for the slag surface (depending on the
size of
the pieces of slag cake) to reach 400 C starting at 700 C is about half the
time
needed to reach those 400 C when starting at 1500 C. The time frame in which
sulfurous emissions may be formed is thus drastically reduced.
[0062] Taking into consideration all of the above assumptions, the expected
sulfurous emissions of the present process amount only to about 1 ¨ 5% of the
generated emissions during classic wet granulation. As a consequence,
compliance with even the most stringent environmental protection laws like the
German TA Luft may be achieved without condensation of the vapors emitted
during the granulation and without the need of a cooling tower. Instead, only
make-up water for the evaporated sulfur-free steam is necessary to allow for
continuous and low maintenance operation.
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Legend:
granulation installation
12 Hot liquid slag
14 slag runner
16 slag caster
18 mold or trough
radar probe
22 first buffer hopper
24 steel balls
26 speed controllable screw conveyor
28 vibrating chute
water sprayer
32 slag cake
34 First impact plate
36 cold runner
38 water jet
granulation head
42 second impact plate
44 water bath
46 belt conveyor
48 crusher
slag particles
52 magnetic conveyor
54 belt conveyor
56 storage area
58 screen
box
62 second hopper
64 water vapor
66 stack
68 conduct
booster pump
72 recirculation pump
